Process for the production of ethylene



United States Patent 2,859,258 PROCESS FOR THE PRODUCTION OF E'I'HYLENEKurt Fischer, Altotling, Upper Bavaria, and Gunter Rummert, Burghausen(Salzach), Germany, assignors to Farbwerke Hoechst- Aktiengesellschaftvormals Meister Lucius & Bruning, Frankfurt am Main, Germany, acorporation of Germany No Drawing. Application October 8, 1954 SerialNo. 461,294

Claims priority, application Germany October 10, 1953 6 Claims. (Cl.260-683) This invention relates to the production of ethylene andparticularly relates to the production of ethylene by the thermalcracking of methane or natural gases which predominately containmethane. This application is a continuation in part of co-pending U. S.patent application Serial No. 432,016, filed May 24, 1954.

Application Serial No. 432,016 is concerned primarily with theproduction of ethylene by the thermal cracking of hydrocarbons whereinthe by-products of the process are recycled for additional treatment toproduce additional quantities of ethylene. The process set forth in thisprior patent application, however, cannot be used with a raw materialsuch as methane. The reason for this is that the thermal conversion ofthe methane molecule requires substantially higher temperatures thanthose necessary for the cracking of the hydrocarbons because of the fargreater thermal stability of methane molecules.

It has heretofore been known to convert methane to acetylene atextremely high temperatures, as, for example, by the use of an electricare. This conversion into acetylene involves a reaction which is undulyexpensive and therefore commercially infeasible. The expense isincurred, in the first place, by the high amounts of energy which areconsumed during the endothermic formation of the acetylene; in thesecond place this acetylene formation requires complex and expensiveapparatus. Furthermore, since the energy must be removed in the form ofhydrogenation heat which is developed during the hydrogenation ofacetylene to ethylene; this exothermic reaction, following the previousendothermic reaction, involves a total waste of energy since the energylevel at the completion of the process is substantially similar to theenergy level at the beginning of the process, even though a great dealof energy has been used during the process. It is, therefore, highlydesirable to effect the cracking between such temperature ranges thatthere is very little formation of acetylene but a substantially largeformation of ethylene.

According to the process embodied in this invention, methane may be usedas the raw material if, prior to the recycle in which the main formationof ethylene takes place, there is provided a recycle operating atsubstantially higher temperatures than the first mentioned recycle.

In the recycle at the higher temperatures the methane is partlyconverted to ethylene and partly to saturated and unsaturatedhydrocarbons having two or more carbon atoms in the molecule. Thesehydrocarbons can then be cracked at lower temperatures.

The raw, cracked gas which is formed in the primary recycle; that is,the recycle operating at the higher temperatures, is separated from theunconverted portion of the methane and the ethylene is then removedtherefrom. The unconverted methane is then returned to the primaryrecycle together with a fresh amount of methane "ice which takes theplace of that consumed during the first reaction. That portion of thecracked gas which remains after the separation of the ethylene isintroduced into the secondary recycle, at the lower temperatures, whereconversion to ethylene takes place in accordance with the process asdescribed in the above-mentioned copending application Serial No.432,016.

The combination of a primary recycle operating at a relatively hightemperature and a secondary recycle operating at a relatively lowertemperature not only serves to simplify the entire process but also actsto save energy.

The saving of energy is efiected by the use of the heating gases, usedin the primary recycle, to heat the apparatus used in the secondaryrecycle, the temperature of these gases having been lowered during theiruse for heating the primary recycle. It has been found that atemperature range of between about 800 C. and 1100 C. is

required for the primary recycle, while a temperature range of betweenabout 600 C. and 800 C. is required for the secondary recycle. Optimumresults have been achieved when the temperature of the primary recycleis between about 950 C. and 1050 C. and when the temperature of thesecondary recycle is between about 700 C. and 750 C.

The recovery of the ethylene from the primary cracking step preferablytakes place by means of adsorption methods. This is necessary in thiscase because the yield is relatively small and results in a so-calledlean gas. It has been determined that adsorption methods are the onlyeconomically feasible methods for recovering the ethylene.

It has been found that the primary cracking step provides the highestyields if the cracking is conducted in the presence of suitablecatalysts. Such catalysts preferably take the form of oxygen-containingmetal compounds wherein the metals are of the second, third and fourthgroups of the periodic system. Among the catalysts which may be used arealuminum oxide, magnesium aluminum silicate, and magnesium aluminummolybdate. In order to obtain optimum results, these oxygen-containingmetal compounds are preferably activated by the addition of smallamounts of heavy metals. Among the heavy metals which may be used arelead, copper, chromium, iron, nickel, cobalt, arsenic, antimony, gold,silver and mercury.

' The primary recycle may be conducted either at atmcspheric,sub-atmospheric or super-atmospheric pressures. More particularly, theprimary recycle is preferably conducted at pressures ranging between oneand five kg./cm. It is preferable, however, to employ a certain amountof superatmospheric pressure, that is,

pressures ranging between one and five kg./cm. in order to simplify theadsorption separation process due to the formation of high molecularreaction products.

The secondary recycle may be conducted under the same pressureconditions as the primary recycle; however, in the secondary recycle theapplication of a super-atmospheric pressure and the employment ofcatalysts are not necessary. The apparatus for conducting the secondaryrecycle, therefore, may be relatively simple and inexpensive.

Example Five cubic meters of methane per hour were introduced at apressure of two kg./cm. into a reaction chamber which was maintained at1050 C. The reaction chamber was filled with a catalyst consisting ofmagnesium aluminum silicate.

The catalyst was activated by a small amount of chromium comprisingabout 3%. The exit gas had the following composition in percent byvolume:

Percent Propylene and butylene 0.2 Propane and butane 2.1 Ethane 1.6Hydrogen 24.3 Ethylene 2.4 Methane 69.4

From this gas mixture the methane and the ethylene were removed by meansof adsorption on active carbon, and the methane was returned to thereaction chamber together with 30.6%. of fresh methane. The entireremaining gas, after separation of the ethylene and methane, was againsubjected to cracking in a coiled tube heated externally to 800 C.andyielded a cracked gas having the following composition:

Percent Propylene and butylene 25.0 Saturated hydrocarbons and hydrogen54.3 Ethylene 20.7

Introduced, 13.33 kg. Obtained, 5.25 kg. Fresh rnethane=100%Ethylene=39.4%

As many apparently widely diflferent embodiments of this invention maybe made without departing from the spirit and scope hereof, it is to beunderstood that the invention is not limited to the specific embodimentshereof, except as defined in the appended claims.

What is claimed in this invention is:

1. A process for obtaining ethylene in high yields comprising the stepsof thermally cracking a gas which is at least predominantly methane at arelatively high temperature ranging between about 800 C. and 1100 C. topartly convert said gas to a mixture of ethylene and saturated andunsaturated hydrocarbons, separating said mixture from the unconvertedportions of the gas, removing the ethylene from the mixture, andsubjecting the remainder ofsaid mixture to a second thermal-- crackingstep at a relatively lower temperature ranging between about 600 C. and800 C. to convertthe said remainder of said mixture at least partiallyto ethylene.

2. The process of claim 1 wherein the unconverted portion of said gas isagain subjected to the thermal cracking at the relatively hightemperature.

3. The process of claim lswherein said relativelyhigh temperaturecomprises a temperature ranging between about 950 C. and 1050" C. andwherein said relatively lower temperature comprises a temperatureranging between 700 C. and '750" C.

4. The process of claim 1 wherein the thermal crack ing of the gas atthe relatively high temperature takes place in the presence of acatalyst comprising an oxygen containing metal compound wherein themetal is selected from the group consisting of the metals of the second,third and fourth groups of the. PeriodicSystem.

5. The process of claim 4 wherein said oxygen-con taining metal compoundis activated by the addition of 1 a relativelysmalliamount of a heavymetal.

6. The process of claim 1 wherein the heat for the second cracking .stepis at least partially provided bythe cooled heating gas from the firstcracking step.

References Cited in the; file of this patent UNITED STATES PATENTS1,524,355 'Haynes et al. Jan. 27, 1925 1,677,363 Olivier July 17, 19282,014,724 Eastman Sept. 17, 1935 2,498,806 Hachmuth Feb. 28, 1950-.

2,597,346 Lefier May 20, 1952 FOREIGN PATENTS 749,916 France Aug. 1,1933 OTHER REFERENCES Frolich et al.: Ind. and Engr. Chem., vol. 27(1935), pp. 10 -9.

1. A PROCESS FOR OBTAINING ETHYLENE IN HIGH YIELDS COMPRISING THE STEPSOF THERMALLY CRACKING A GAS WHICH IS AT LEAST PREDOMINANTLY METHANE AT ARELATIVELY HIGH TEMPERATURE RANGING BETWEEN ABOUT 800*C. AND 1100* C. TOPARTLY CONVERT SAID GAS TO A MIIXTURE OF ETHYLENE AND SATURATED ANDUNSATURATED HYDROCARBONS, SEPARATING SAID MIXTURE FROM THE UNCONVERTEDPROTIONS OF THE GAS, REMOVING THE ETHYLENE FROM THE MIXTURE, ANDSUBJECTING THE REMAINDER OF SAID MIXTURE TO A SECOND THERMAL CRACKINGSTEP AT A RELATIVELY LOWER TEMPERATURE RANGING BETWEEN ABOUT 600*C. AND800C. TO CONVERT THE SAID REMAINDER OF SAID MIXTURE AT LEAST PARTIALLYTO ETHYLENE.